Two-way radio communication systems are well known. Examples include cellular telephone systems and trunked mobile radiotelephone systems. Such systems typically have utilized equal transmission bandwidths and substantially similar output power levels in both directions of transmission (portable-to-base and base-to-portable). Recently, asymmetrical two-way radio communication systems have come into existence. Asymmetrical systems do not use equal bandwidths or equal output power levels in both directions of transmission. Typically, asymmetrical systems use high power and high bandwidth in the base-to-portable direction, while using low bandwidth and low power in the portable-to-base direction. High bandwidth in the base-to-portable direction provides high data capacity in the base-to-portable direction, which is required, for example, in a high capacity selective call system.
Low bandwidth in the portable-to-base direction is advantageous in systems having battery powered portable units, which require low power operation to achieve long battery life. Low bandwidth is used to extend the transmission range of a low power transmitter to match the range of the high power base-to-portable direction. Low bandwidth necessarily implies low data capacity per channel, which in turn can necessitate multiple channels, e.g., the use of Frequency Division Multiple Access (FDMA), in the portable-to-base direction to increase capacity by allowing multiple users to communicate simultaneously on multiple narrowband channels.
Multiple narrowband channels can operate successfully only if the carrier frequencies of the narrowband channels can be controlled accurately enough to prevent overlap and resultant co-channel interference. Accurate frequency control could of course be accomplished by utilizing precision oscillators, but such oscillators would increase the cost, size, and power consumption of the portable unit, thereby substantially defeating the advantages of using the narrowband channels in the first place.
Thus, what is needed is a method and apparatus for controlling the carrier frequency of a narrowband portable-to-base channel without requiring precision oscillators. A reliable method and apparatus are needed that do not substantially increase the size, cost, or power consumption of the portable unit.